Field of the Disclosure
The present disclosure relates generally to wireless communication and, more particularly, to downlink transmissions in unlicensed frequency bands of a wireless communication system.
Description of the Related Art
Unlicensed frequency bands are portions of the radiofrequency spectrum that do not require a license for use and may therefore be used by any device to transmit or receive radiofrequency signals. For example, the industrial, scientific, and medical (ISM) radio bands are portions of the radio spectrum that are reserved internationally for unlicensed communication. The ISM radio bands include bands with a center frequency of 2.4 GHz and a bandwidth of 100 MHz, a center frequency of 5.8 GHz and a bandwidth of 150 MHz, and a center frequency of 24.125 GHz and a bandwidth of 250 MHz, among other frequency bands. Unlicensed frequency bands can be contrasted to licensed frequency bands that are licensed to a particular service provider and may only be used for wireless communication that is authorized by the service provider. Wireless communication devices that transmit or receive signals in unlicensed frequency bands are typically referred to as nodes. For example, the base stations or access points that provide wireless connectivity to a network and the user equipment or other devices that access the network over an air interface to the base stations or access points may be referred to as nodes in the wireless communication system.
Wireless communication systems that utilize unlicensed frequency bands, such as Wi-Fi systems, are prone to a “hidden node problem.” For example, if two user equipment are within range of the same access point, but are too far apart to be aware of each other, the two user equipment are “hidden” from each other. Access points or base stations can also be hidden from each other by distance or intervening obstructions. Nodes that are hidden from each other cannot coordinate transmission and reception of packets, e.g., to force time-sharing between the two nodes. Packets transmitted by nodes that are hidden from each other may therefore collide at a receiving node, which can only decode one packet at a time. Consequently, packets intended for the receiving node may be missed or lost if they collide with other packets transmitted by a hidden node. The hidden node problem can be exacerbated by the presence of obstructions between the stations for the access points. For example, building penetration losses are typically on the order of 11-20 dB. Consequently, an indoor access point may be hidden from an outdoor base station even though they may be physically proximate to each other. Similarly, two user equipment in the same building may be hidden from each other if they are separated by one or more walls, doors, or other obstructions within the building.
A carrier sense multiple access (CSMA) protocol may be used to detect or avoid collisions that may be caused by the hidden node problem. In CSMA, a transmitting node monitors a channel in the unlicensed band to determine whether it is currently being used for other transmissions and only transmits if the channel is unoccupied. The CSMA protocol may be enhanced using a request-to-send/clear-to-send (RTS/CTS) protocol. The RTS/CTS protocol attempts to reduce collisions by allowing a transmitting node to send an RTS frame that indicates that the transmitting node would like to transmit information to a receiving node if the transmitting node detects a clear unlicensed channel. If the receiving node also determines that the unlicensed channel is clear, the receiving node replies with a CTS frame that indicates that the transmitting node is free to transmit information on the unlicensed channel for a time interval. Other nodes that detect the CTS frame are to refrain from transmitting on the unlicensed channel during the time interval indicated in the CTS frame. However, carrier sensing protocols may not effectively prevent collisions of downlink transmissions if some or all neighboring access points or base stations do not “hear” the RTS or CTS frames transmitted by the user equipment.